At the Hilton San Francisco Union Square hotel, a 1960s-era multizone air handler was beyond its recommended service life, and maintenance requirements were increasing. Additionally, the economizer dampers, zone dampers, and linkage were rusted. Deteriorating and corroded cooling-coil and steam-coil surfaces reduced system cooling and heating performance, making the constant-volume system far less efficient over time.

“We were trying to control temperature by running steam and chilled water with one coil bank,” Bob Woodfin, the hotel’s chief engineer, said.

The 47,800-cfm-capacity unit with double-wide-double-inlet housed fan assembly served four ballroom foyers on the second floor. Most worrisome for Woodfin was how to get the large fan, located deep in the 12th-floor mechanical room, out and another one in.

“There aren’t a lot of alternatives for a fan like this,” Woodfin said. “Although we were looking primarily for easy retrofit, the additional benefit of fan redundancy certainly became a significant advantage.”

Original Solution Poses Problems Space constraints made installing a new unit the same size as the old one impractical. Although the mechanical room is large—24-ft high—there is little room for knockdown or staging. All new equipment would have to come in through a standard 3-ft-wide door and then be moved along a narrow walkway.

“Our original thought was to specify four small air-handling units with separate controls because we needed temperature control for each zone,” Woodfin said. “This would allow us to replace the steam heat with hot-water reheat to solve the problem of regulating temperature in the different zones. However, four air handlers were still fairly large. Units would have to be either shipped in pieces to the Hilton or disassembled at the site (and) then reassembled in place because of the restricted access to the mechanical room.”

Alternative Found “Bob Woodfin told us that he really needed to get this air handler replaced, but was looking for someone to come up with an innovative idea that could meet his budget,” Chris Barger, project manager, Allied Heating and Air Conditioning Company Inc., San Rafael, Calif., said. “I knew the four-unit approach was cost-prohibitive, having previously done other similar projects, so I pursued a different way to do the project.”

A few years earlier, Barger had learned of FANWALL TECHNOLOGY from Huntair Inc., a CES Group company. FANWALL TECHNOLOGY is a modular array of identically sized fans and motors that operate in parallel to create the same airflow rate as a single larger fan sized for the same duty.

Barger met with Shaun Webster, sales engineer for Norman S. Wright Co., the local manufacturers’ representative for Huntair. Barger’s idea was to gut the cabinet of the air handler, retain the 20-ft-by-10-ft-by-30-ft chassis, and build it up to meet Woodfin’s expectations for performance.

If a large fan or motor like the one previously at the Hilton fails, the result is 100-percent failure of the air-handling system.

“But if there’s a failure in the array of FANWALL TECHNOLOGY fans or motors, the integral variable-frequency drive simply adjusts the operating speed of the remaining enabled fans in the array to produce the required airflow and static pressure until the failed unit can be replaced,” Webster said.

A FANWALL system consists of cube-shaped cells, each housing a fan, a motor, and electrical connections. The number of fans, wheel diameter, wheel width, and operating speed (revolutions per minute) are customizable. In the Hilton’s 12th-floor mechanical room, eight FANWALL cells (stacked two high by four wide) provide the same 48,000 cfm as the old air handler. Each 550-lb cell measures 29 in. deep, small enough to be loaded onto a cart and wheeled through the 36-in.-wide mechanical-room door.

The steam-heat issue was resolved by running hot-water supply service to a variable-air-volume terminal unit in each of the four zones and enabling the units with hot-water reheat. This made possible the removal of the main heating coil for a more aerodynamic flow of air into the replacement chilled-water coil. The result was vastly improved cooling-coil performance. The cooling-coil bank includes copper fins and stainless-steel casings, while the dampers and linkages are made of stainless steel for improved durability and longer service life.

“The FANWALL approach was a very attractive way of meeting project performance objectives, while at the same time falling within the budgeted costs,” Barger said. “It also required minimal system downtime and virtually no building structural demolition. ... The FANWALL TECHNOLOGY cells made it easy to accomplish the retrofit while providing the required system cfm and static pressure, at the same time reducing energy costs.”

How It Was Done “I had never heard of FANWALL TECHNOLOGY, but I liked the idea that the solution would keep us within our budget,” Brian Mork, director of property operations, Hilton San Francisco, said. “The labor costs for demolition and assembly we were looking at for the other alternatives would have significantly increased the cost of the project.”

The entire project was completed in stages over eight days, with the intention to minimize any disruption to hotel guests. Installation of the FANWALL system took a fraction of that time.

“We started at about 8 o’clock in the morning with three guys and finished by 10 a.m.,” Josh Mauer, sheet-metal foreman, Allied Heating and Air Conditioning, said.

The most time-consuming part was demolishing the 8-ft-tall fan and associated ductwork and coils. Preconstruction was a priority, and each step was scheduled to ensure each trade completed its work on time.

“Each fitting had to be staged like a puzzle as we built ourselves out,” Barger said.

The rest of the time was devoted to rigging six chilled-water coils, connecting ducts, and adding control dampers and hot-water reheat coils.

Careful attention to balance within each fan wheel significantly reduces vibration. In the case of the Hilton, this eliminated the need for a concrete inertia base and spring isolation. Small fans operating at relatively high speeds produce less troublesome low-frequency noise than larger fans operating at lower speeds sized for the same airflow and static pressure. Noise reduction is accomplished using the Coplaner Silencer, which surrounds the fan and motor with acoustically absorbent material to greatly reduce airborne noise at the source.

“If you went into the fan room where the Hilton’s air handler was operating, it was so loud that you could barely hear yourself talk,” Webster said. “After FANWALL TECHNOLOGY was installed, I brought all the Hilton engineers in there for training while the system was operating at 100 percent of its airflow capacity, and we were able to have a normal conversation.”

Besides being noisy, the old air handler operated at a constant speed. The FANWALL TECHNOLOGY system combines a variable-frequency drive and control system to adjust fan speed automatically, depending on the concurrent system demand. For example, if just one of the areas being served is occupied, the fans are slowed to deliver only the amount of airflow needed in that zone.

Other HVAC Problems Tackled According to Webster, energy efficiency also was improved by removing “a lot of the junk in the air stream.” For example, the transition ductwork downstream of the fan included a restriction that was causing a large amount of static-pressure loss. This section of ductwork was replaced with duct with a wider opening and no restrictions, improving airflow dynamics.

“Energy is always a big criterion for me,” Mork said. “In all, the FANWALL TECHNOLOGY system saved about 137,000 kwh of energy over the first year compared to the old air-handling system.”

As a result, Mork added, the hotel received a $13,000 rebate from Pacific Gas and Electric Co., the local utility.

Ventilation in the ballroom foyers now is comfortable and easy to control. And hotel engineers now have a new way of approaching air-handling retrofits.

Information and photographs courtesy of Huntair Inc.

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